add option to allow ssh to bypass HTTP proxy

In my company, HTTP access (to 80, etc.) has to go through the HTTP proxy, but access to ssh is allowed directly. jclouds used to work with that but I think in the last 8 months or so this was changed and socket:// connections now also try to go through the HTTP proxy. Assuming that works for others, I'm suggesting to add this option, with the default behaviour remaining the way it is now.